To suppress damage to a cylinder block due to stress concentration such as the occurrence of a crack in a cylinder block.

A cylinder block 1 is equipped with a water jacket 13 formed so as to surround the periphery of a cylinder bore #1 and opened in the upper deck 12 of the cylinder block 1 and with a partition wall 18 between the water jacket 13 and cylinder bore #1. The partition wall 18 includes a uneven thickness part 18a having a locally increased wall thickness at a prescribed position in a circumferential direction of the cylinder bore #1. In the cylinder block 1, the uneven thickness part 18a is set to decrease the uneven thickness amount T as being situated near the upper deck 12 side in an axial direction L of the cylinder bore #1.